1. Field of the Invention
The present invention relates to a resin composition for an optical disc for use in information recording, etc., and an optical disc using it.
2. Description of the Related Art
As a material for a substrate of a conventional optical recording medium (optical disc), use is made of glass, acrylic resin, polycarbonate resin, epoxy resin, polyolefinic resin, etc.
Generally, an optical disc comprises a substrate with optical guide grooves, pits, etc., for achieving a higher density recording and an underlying layer, recording layer, reflective layer, etc., formed over the substrate.
The substrate as set out above is usually prepared by the following four methods.
The first method comprises injecting a melting resin into a mold equipped with a stamper having projection spots corresponding to the shape of guide grooves and solidifying it. As a resin using this method, use is made of a thermoplastic resin, such as an acrylic resin, polycarbonate resin and polyolefinic resin.
The first method is excellent in quantity production, but presents the following problems.
An optical disc 10 as produced by the first method shown in FIG. 1A has a birefringence as its optical characteristic varied nonuniformly and undergoes a deformation in a substrate 11 originating from the nonuniform thickness of the substrate and an axial runout in a surface of the substrate. Further, due to the gas permeability and hygroscopicity of the resin per se, an underlying layer 12 and recording layer 13 are deteriorated and a deformation in the substrate 11 occur. When, for example, the underlying layer 12 and recording layer 13 are to be formed by sputtering over the substrate, the substrate 11 is softened owing to the low heat-resistance of these resins used and suffers a drop in an adhesion between the substrate 11 and the underlying layer 12. As a result, the recording layer 13 on the underlying layer 12 wrinkles or cracks. Further, as these resins usually contain an internal mold release agent so as to improve a separation from a stamper, the substrate 11 and underlying layer 12 are lowered in their adhesion. The use of the first method requires a very large equipment.
The second method known as a so-called 2P method comprises, as shown in FIG. 1B, placing a stamper and transparent substrate, comprised of glass or transparent resin, in a mold at a predetermined interval, casting an ultraviolet radiation-curing type resin into a spacing between the stamper and the transparent substrate, emitting an ultraviolet radiation onto the resin through the transparent substrate 14 to allow the resin to be joined to the substrate by causing the resin to cure, and then forming guide grooves or pits 16 on the cured resin 15. In this method, an acrylic resin, epoxy resin, etc., are employed as the ultraviolet radiation curing type resin.
The 2P method is inferior to the injection molding method in quantity production. Since, according to this method, an unreacted low molecular component, such as a monomer, is left at the unexposed portions of the ultraviolet radiation curing type resin, it is necessary to remove them by washing. The unreacted component is left unremoved even if washing is done. In the formation of the recording layer by sputtering on the substrate, the unreacted resin component and the substance having low molecular weight are evaporated, hindering the formation of the recording layer and hence taking lots of time in its formation. The evaporated unreacted resin component and low molecular weight substance are deposited on the inner surface of the equipment and frequent maintenances have to be done in the equipment. For a cured resin of lower heat resistance, the recording layer wrinkles or cracks as in the first method. The substrate as manufactured by the 2P method, by forming a film carefully, prevents a wrinkle or a crack on the recording layer, but the recording layer wrinkles or cracks upon a passage of a longer period of time.
The third method comprises setting a stamper having a predetermined projection pattern in a mold, casting a reactive resin, such as an epoxy resin, into a cavity of the mold and curing it through a reaction.
In this casting method, it takes a longer time to achieve a curing step so as to prevent a drop in optical characteristic originating from an internal deformation caused upon the reaction of the reactive resin. The method is not better in quantity production. In order to improve a separation from the stamper, etc., a drop in a polar group in the resin and addition of an internal mold release agent to the resin are accomplished, lowering an adhesion between a substrate and a recording layer. The resin, being of a lower heat resistance type, wrinkles or cracks in the recording layer as in the preceding method.
The fourth method comprises forming a resist layer on the surface of a glass substrate, either recording information in the form of grooves and pits directly on the resist layer or forming a resist layer on the surface of a glass substrate 17 as shown in FIG. 1C, exposing the resist layer using a mask, developing it and eliminating those resist layer portions corresponding to the grooves and pits, and etching the glass with the resist layer used as a mask to provide grooves and pits 18 directly on the glass substrate.
The fourth method can obtain a substrate of an excellent characteristic, but it involves a complex manufacturing process and lower quantity production.
If an optical disc substrate is manufactured by the forementioned method with the use of a conventional resin composition, a crack or a wrinkle occurs on the surface of the recording layer overlying the substrate, lowering the recording characteristic of an optical disc obtained. A substrate, even if exhibiting an excellent recording characteristic, results in a lowered quantity production.